1. Field of the Invention
This invention relates to a method and apparatus for recovering ductility of a cold rolled metal strip.
In producing a metal strip, tensile strength and ductility of the metal strip are controlled by changing the reduction of rolling, and the temperature and time of heat treatment. Recently, extremely thin metal strip of high tensile strength have been produced by a double reduce method of rolling a metal strip with large reduction after annealing instead of conventional tempering.
A metal strip manufactured by this method has a high tensile strength, but ductility and workability are low because of work hardening by rolling with a large reduction. This sometimes causes breaking of the material in a can making process, particularly in the flanging process. Can making is a major field in which thin metal strips are employed. Workability in can making does not have to be as large as in deep drawing and a small increase in workability is good enough. It has been found that an increase in ductility by a small percentage in a thin metal strip helps reduce the trouble. However it is not desirable on the product and installation costs to provide an annealing process for small increases in ductility.
2. Description of the Prior Art
The patent publication of the application Nos. 41-12,292 and 47-19,856 in Japan show ductility of a cold rolled metal strip can be recovered by bending it repeatedly 50 to more than 100 times with a roller leveler. In these methods, alternate tensile and compressive strains by bending result in softening the material. These disclosures have the disadvantage that a large space is necessary to install a leveler with a number of rollers for continuous production and further, scratches arise on the surface of a metal strip because it is difficult to rotate many rollers of the roller leveler at the same speed. The metal strip passes through a curing process after printing. Therefore, the ductility of the metal strip subjectd to heat treatment even decreases due to scratches on the surface and micro fractures in the metallic crystal when shape defects of the metal strip are corrected by a leveler as mentioned in the references.
An object of the present invention is to provide a method of recovering ductility of a cold rolled metal strip using a less expensve and smaller equipment.
Another object of the invention is to provide a method of recovering ductility without causing scratching on the surface of a metal strip. Further object is to provide a method by which the effect of recovering ductility is maintained even after heat treatment.